A motor drive system, in one known form, comprises an AC source supplying three-phase AC power to a variable frequency inverter (VFI). The VFI includes an AC/DC converter connected by a DC link to a DC/AC converter. The DC/AC converter may comprise a pulse width modulated inverter using insulated gate bipolar transistors (IGBTs).
In general, the output of the VFI is modulated at a carrier frequency ranging from one kHz to 20 kHz. The higher carrier frequency, particularly in small sized VFIs up to 75 kilowatts, along with the fast rise time of the IGBTs employed results in high dv/dt. This, in turn, yields non-trivial common mode or ground currents. If the distance between the motor and the VFI is long and there exists a mismatch in the cable and motor surge impedance, then there is voltage amplification at the motor terminals. Hence, the dv/dt effects in conjunction with larger cable capacitance enhances the common mode currents.
In practice, the over voltage at the motor terminals, which depends upon the distance between the motor and the VFI as well as the impedance mismatch between the cable and the motor surge impedance, can reach as high as twice the DC bus voltage. In certain cases, due to overlap of the modulating pulses, the peak transient appearing across the motor windings can be as high as three times the DC bus voltage. The high rate of rise of voltage pulses in the range of a few hundreds of nanoseconds give rise to ground currents due to cable capacitance to ground and motor winding capacitance to ground.
The over voltage at the motor terminals due to long lead lengths can cause premature insulation failure in the motor. There exists a non-trivial parasitic capacitance between the stator and the rotor, which is instrumental in creating a charge path between the stator and the rotor. Due to large dv/dt of the common mode voltage, and the above mentioned parasitic capacitance, the rotor develops a voltage similar to a charge pump. When this voltage exceeds the breakdown voltage of the thin lubricant film between the inner and outer rings of the bearing, there is a miniature flashover. This causes pitting in the bearings and is the main reason for premature bearing failure. If not properly mitigated, high frequency ground currents can also create interference with the power system ground and affect other equipment on the power system. This phenomenon contributes to conducted EMI.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.